A study on the effects of seismicity on subsidence in foreland basins: An application to the Venice area

Foreland basins are flat elongated areas occurring along subduction and collision zones worldwide. We show that, in such basins, subsidence can be induced by earthquakes generated along bordering thrust faults due to coseismic displacement, postseismic displacement and liquefaction-induced compaction.As an example, the potential effects of earthquakes on the subsidence of Venice, which is located in the Po Plain foreland basin, are discussed. It is generally assumed that natural subsidence of Venice is continuous and that subsidence rates are rather constant through time. However, catastrophic pulses of subsidence cannot be ruled out as taught by the sudden disappearance of the island of Malamocco at the beginning of the XII century.The results of numerical models specifically run suggest that the risk of subsidence accelerations in Venice due to coseismic displacements is negligible. Modelling results from literature suggest that postseismic subsidence could be of the order of 1 cm. Although the effects of a single event should be improbably detectable, such a subsidence is not a priori negligible considering the number of seismogenic sources located within 100 km from the town.Historical sources are utilized to discuss the feasibility of liquefaction-induced subsidence in Venice. It is shown that the destruction and sinking of ancient Malamocco is roughly coincident with a strong earthquake cycle that was associated to phenomena that can be explained with liquefaction of sandy layers. Although the historical documents do not permit to establish a clear causal link between the earthquake and land subsidence, it is concluded that liquefaction-induced subsidence cannot be ruled out as a potential source for local subsidence acceleration.     

Our distorted view of magnetars: application of the resonant cyclotron scattering model

The X-ray spectra of the magnetar candidates are customarily fitted with an empirical, two component model: an absorbed blackbody and a power-law. However, the physical interpretation of these two spectral components is rarely discussed. It has been recently proposed that the presence of a hot plasma in the magnetosphere of highly magnetized neutron stars might distort, through efficient resonant cyclotron scattering, the thermal emission from the neutron star surface, resulting in production of non-thermal spectra. Here we discuss the Resonant Cyclotron Scattering (RCS) model, and present its XSPEC implementation, as well as preliminary results of its application to Anomalous X-ray Pulsars and Soft Gamma-ray Repeaters. N.R. is supported by an NWO Post-Doctoral Fellowship. S.Z. thanks the Particle Physics and Astronomy Research Council, PPARC, for support through an Advanced Fellowship.     

Detecting filaments in the ultra-high energy cosmic ray distribution

We propose and test new statistical tools to study the distribution of cosmic rays based on the use of the minimal spanning tree. The method described is particularly sensitive to filamentary structures, as those expected to arise from strong sources of charged cosmic rays which get deflected by intervening magnetic fields. We also test the method with data available from the AGASA and SUGAR surface detector arrays.     

Palaeoenvironmental turnover across the Palaeocene/Eocene boundary at the Stratotype section in Dababiya (Egypt) based on benthic foraminifera

The Global Stratotype Section and Point for the Palaeocene/Eocene (P/E) boundary was defined at Dababiya Quarry (Egypt) at the base of the carbon isotope excursion (CIE). We present the first detailed analysis of Palaeocene–Eocene benthic foraminifera from Dababiya, in order to infer the palaeoenvironmental turnover across the P/E boundary. At Dababiya, the CIE coincides with a major turnover in foraminiferal assemblages; the last occurrence of Angulogavelinella avnimelechi, at the base of the CIE, may be correlated to the main phase of extinction of deep-sea benthic foraminifera. Benthic foraminifera indicate that stressful conditions such as oxygen deficiency, carbonate dissolution, and changes in food supply, persisted at the sea floor over most of the CIE interval. The main phase of recovery of benthic foraminifera is recorded c. 250 cm above the P/E boundary, and it may be linked to increased productivity and oxygenation at the sea floor.     

Detection of Flow Distortions and Systematic Errors in Sonic Anemometry Using the Planar Fit Method

We describe the coordinate transformations that can be used to convert the velocity components measured by a set of sonic anemometers with time-dependent tilt fluctuations into a single, time-independent coordinate system. By applying the planar fit method (PFM) to each anemometer dataset, it is possible, for planar flows, to locate the flow plane at each measurement point and compare its orientation with the topography. Installation on a ship is also considered. An application of this method to intercomparison data has led to the detection of an instrument error due to a misalignment between the assembly of the sonic transducers and the anemometer pedestal. If this error occurs, pedestal levelling does not guarantee that measurements are unbiased. A correction method is proposed and the results of two experiments are shown. Flow planarity at different levels and flow distortion caused by the mast are highlighted. The influence of the error on the evaluation of the Reynolds stresses using PFM or the double rotation method and the triple rotation method is discussed and the tilt corrected stresses calculated using the three methods compared.     

Tepexpan Palaeoindian site, Basin of Mexico: multi-proxy evidence for environmental change during the late Pleistocene–late Holocene

The Tepexpan Palaeoindian skeleton was discovered in 1947 close to the former Lake Texcoco margin, in the Basin of Mexico. The find has been the object of considerable interest and discussion over the last 60 years regarding its real age and archaeological interpretation. Here we report new AMS radiocarbon dates associated with the sedimentary succession at Tepexpan with ages between 19,110 ± 90 and 612 ± 22 ¹⁴C years BP and a new uranium-series date for the skeleton with an age of 4700 ± 200 years BP that indicates a mid Holocene age. The sedimentary succession was studied in detail using: stable isotopes, diatoms, organic geochemistry and tephrochronology. The multi-proxy evidence suggests large changes around the margins of Lake Texcoco in terms of the balance between aquatic and terrestrial plants, C₃ and C₄ plants, saline, alkaline and freshwater conditions, volcanic activity, marginal reworking of lake sediments and input from the drainage basin through the late Pleistocene–late Holocene. These changes had large impacts on the prehistoric human populations living by the lake shores since the late Pleistocene in the Basin of Mexico.     

Bronze Age paleohydrography of the southern Venetian Plain

The Bronze Age paleohydrography of the distal Adige and Po alluvial plain (northeastern Italy) is notable for its relations with protohistoric human activities in this area. This paper regards the stratigraphy and petrography of the Saline–Cona alluvial ridge, upon which the Saline, Sarzano, and Cantarana Bronze Age sites lie, and the petrography of Fratta alluvial ridge, upon which the Frattesina complex (Bronze–Iron Age) lies. Sand analyses indicate the Po River as the source for sediments underlying the alluvial ridge that runs through Fratta Polesine, Rovigo, Sarzano, and Cona. Radiometric ages indicate that the branch of the Saline–Cona ridge was formed by the Po River between the second half of the 3rd millennium B.C. and the end of 2nd millennium B.C. This ridge represents the maximum northward expansion of the Po alluvial system, through the same area of coastal plain crossed by the Adige and Brenta paleochannels. This paleohydrographic setting implies that fluvial connections between the Central Po Plain settlements, the Venetian Plain and Alps were relatively less complex in the Early and Middle Bronze Age than in the Late Bronze Age, when the terminal reach of the Po River was separated by the Adige River by hundreds of km² of swampy terrain. © 2009 Wiley Periodicals, Inc.     

Characterization of the Eva Verda basin aquifer (Saint Marcel, Aosta Valley, Italy) through geochemical and geostructural methods and analysis

The groundwater system of the Eva Verda basin (Saint Marcel Valley, southern side of the middle Aosta Valley, Italy) has many springs that can be used as sources for drinking water. This area is near the disused Servette mine, which can be a pollutant source (metals and sulfides) for the springs located downhill. Aquifer characterization was done using a multidisciplinary approach: geostructural, lithological, hydrogeological and geochemical. In particular, the geostructural analysis showed that the preferential water-flow direction is controlled by tectonics and that it has a trend along the slope toward the downhill springs. The mine drainage flow direction is in agreement with this trend and can pollute the springs. Chemical analysis revealed three water groups: (1) SO ₄ ^2- –Ca²⁺–Mg²⁺ rich water (mine drainage), (2) HCO ₃ ^- –SO ₄ ^2- –Ca²⁺ rich water and (3) HCO ₃ ^- –Ca²⁺ rich water (freshwater). The second group of water results from the different percentage mix between the first and the third waters. The low percentage of mine polluted water demonstrates that there is a high dilution and low pollution of waters that can be exploited for drinking.     

Bentho-planktonic evidence from the Austrian Alps for a decline in sea-surface carbonate production at the end of the Triassic

A high-resolution micropalaeontological study, combined with geochemical and sedimentological analyses was performed on the Tiefengraben, Schlossgraben and Eiberg sections (Austrian Alps) in order to characterize sea-surface carbonate production during the end-Triassic crisis. At the end-Rhaetian, the dominant calcareous nannofossil Prinsiosphaera triassica shows a decrease in abundance and size and this is correlated with a increase in δ¹⁸O and a gradual decline in δ¹³C_carb values. Simultaneously, benthic foraminiferal assemblages show a decrease in diversity and abundance of calcareous taxa and a dominance of infaunal agglutinated taxa. The smaller size of calcareous nannofossils disturbed the vertical export balance of the biological carbon pump towards the sea-bottom, resulting in changes in feeding strategies within the benthic foraminiferal assemblages from deposit feeders to detritus feeders and bacterial scavengers. These micropalaeontological data combined with geochemical proxies suggest that changes in seawater chemistry and/or cooling episodes might have occurred in the latest Triassic, leading to a marked decrease of carbonate production. This in turn culminated in the quasi-absence of calcareous nannofossils and benthic foraminifers in the latest Triassic. The aftermath (latest Triassic earliest Jurassic) was characterised by abundance peaks of “disaster” epifaunal agglutinated foraminifera Trochammina on the sea-floor. Central Atlantic Magmatic Province (CAMP) paroxysmal activity, superimposed on a major worldwide regressive phase, is assumed to be responsible for a deterioration in marine palaeoenvironments. CAMP sulfuric emissions might have been the trigger for cooling episodes and seawater acidification leading to disturbance of the surface carbonate production at the very end-Triassic.